The systems and methods disclosed relate generally to wireless power transfer and more particularly to the conversion of wireless power to direct current (DC) power.
The increased performance and decreased power requirements of integrated circuits has resulted in an explosion of devices that operate completely independent of wires or power cords. These “untethered” devices range from cell phones and wireless keyboards to building sensors and active Radio Frequency Identification (RFID) tags. Engineers and designers, however, continue to face limitations in the storage capacity of portable power sources, primarily batteries, which can be used to provide power to these devices. Battery technology, and particularly battery storage capacity, has only been growing at a meager 6% per year. Even with the use of power-efficient integrated circuits, the storage capacity of today's batteries is unable to keep up with the power requirements of many untethered device applications.
One approach to address the limitations in today's battery technology has been to harness sufficient energy or power from the environment (e.g., ambient power) or from a transmitter (e.g., radio frequency (RF) power) for use in the untethered device. The harnessed power would then be converted to a DC power to directly power an untethered device or to recharge a battery or other storage component. Directly powering an untethered device enables the device to be constructed without the need for a battery. Recharging a storage component could increase the time of operation of the device. Other preferred benefits include the untethered device being able to be used in a wide range of environments, including harsh and sealed environments (e.g., nuclear reactors), to be inexpensive to produce, to be safe for humans, and to have a minimal effect on the basic size, weight and other physical characteristics of the untethered device.
In many instances, however, the amount or level of energy or power available for harnessing is very low (e.g., −20 dBm or lower). In such instances, a wireless power receiver used to convert the incident power to a DC voltage typically uses a resistive load such that even low levels of incident power produce a large DC voltage. For example, the DC voltage varies proportionately with the incident power This approach, however, does not result in a constant or reliable DC voltage (or DC power) that is suitable to operate an untethered device. Although a lower DC voltage may be desirable to efficiently convert low power levels, many untethered devices require large DC voltages to operate. Moreover, this approach is effective only when the incident power is characterized by a relatively narrow frequency spectrum, thus limiting the ability to harness or collect power in areas where the incident power has is associated with a wide frequency spectrum. In addition, at low power levels, it is also desirable that the conversion circuitry and/or the battery charging circuitry in the wireless power receiver operate such that the net charge or power delivered to the battery is increased. In other words, it is desirable to reduce or minimize the amount of reverse current that is drained through the wireless power receiver from the battery during the charging process.
Thus, a need exists for a wireless power receiver that can operate at low levels of incident power, can convert incident power characterized by a wide frequency spectrum to DC power, can efficiently recharge a battery in an untethered device, and/or can efficiently operate an untethered device.